Present methods of measuring the optical power of the cornea actually involve calculating the image distance and radius of curvature of the cornea. This is usually accomplished by using an instrument called a keratometer. Keratometers, the most common of which are made by Bausch & Lomb, American Optical, Keller, Haig Strait and Terry Operating Keratometer, project an image of a known size onto the cornea and the size of the reflected image is measured to determine the magnification (actually minification since the cornea is convex). Because the object is a known distance from the anterior corneal surface, the image distance can be calculated. The radius of curvature of the anterior corneal surface can then be calculated and the dioptric power of the cornea determined.
The basic assumption in calculating the corneal radius of curvature by keratometry is that the anterior corneal surface is a perfectly corrected optical surface. The cornea, however, is not a perfect optical surface and, in fact, is rather irregular. An imperfectly corrected optical system like the normal anterior surface of the cornea will not actually have a focal point, but a circle of least confusion, i.e., the cornea has spherical aberration. As a result, standard keratometers will give an incorrect radius of curvature for the cornea, and in general, will give a steeper radius of curvature than the actual radius of curvature of the cornea. This concept is important in understanding the optical power of the cornea following refractive techniques such as radial keratotomy, epikeratophakia, keratophakia, and keratomileusis, and probably accounts for many of the inaccuracies obtained when using formulas to determine intraocular lens power.
Immediately following radial keratotomy and other refractive procedures, the cornea becomes an even more imperfect optical system, and the error in calculating the image distance and radius of curvature of the cornea will be even larger. To determine the actual effective optical power of the cornea (effective optical radius of curvature), the image distance must actually be measured, not calculated. A need exists, therefore, for a method and device for measuring the actual image distance from the anterior corneal surface.